A family in which manic-depressive and hyperkalemic familial periodic paralysis, a neurological disease, appear to assort together is studied. The hypothesis that there is an autosomal dominant mode of inheritance for manic-depressive illness is tested by attempting to demonstrate a linkage with familial periodic paralysis, which is known to have autosomal dominant transmission. The efficacy of lithium carbonate in preventing attacks of familial periodic paralysis is tested in a double-blind trial.